The present invention relates to a perfecting printer for printing images on both of obverse and reverse faces of a sheet to perform a double-face printing operation, and more particularly to a perfecting printer having a control unit for controlling the double-face printing operation of printer.
As disclosed in Japanese Laid-Open Patent Publication No. 59(1984)-179354, there has been already known a perfecting printer such as a laser printer capable of performing a perfecting (double-face) printing operation in which data such as character data, graphic data or the like are printed on both of obverse and reverse faces of a sheet. When the double-face printing operation is carried out for plural sheets in the perfecting printer, each of the sheets must be successively discharged with the printed face thereof placed downwardly in order to sequentially print a series of pages for a short time period. The sheet discharging operation as described above is generally called "face-down discharging operation".
Further, in order to print images on both of the obverse and reverse faces of a sheet, it is required to pass the sheet through a recording unit of the printer two times while the face to be printed is reversed each time. Therefore, the perfecting printer is provided with a sheet path change-over or switching mechanism and a sheet reversing path.
In order to miniaturize the housing of the perfecting printer and the sheet feed path thereof and to simplify a control operation of the sheet switching mechanism, the following construction has been proposed for the perfecting printer adopting the face-down discharging method: A face-down discharge tray is disposed at an upper portion of the housing of the printer, and the sheet feed path which extends to the discharge tray is branched from the sheet reversing path and bent in a U-shape at a suitable position in the printer.
In a case where the sheet feed path having the above construction is utilized in the printer, the face-down discharging operation inevitably requires the following printing operation: The reverse face (second page) of a sheet is firstly printed and then the obverse face (first page) is printed. Accordingly, print data for at least two pages must be entirely stored into a memory to print images on both of the obverse and reverse faces of a sheet.
In this perfecting printer, even if no print data exists for the reverse face of a sheet, an printing operation for the obverse face of the sheet is commenced after completion of a printing operation for the reverse face of the sheet, which would be carried out if the print data for the reverse face exists. Therefore, a period required for printing an image on one face of a sheet because no print data for the other face exists is the same as the period for printing images on both faces of the sheet because the print data for the obverse and reverse faces of the sheet exist. Accordingly, the perfecting printer as described above has an inefficient printing operation.
The above event where there is no print data for the reverse face of a sheet occurs, for example, in the following case. When a series of sentences are printed on obverse and reverse faces of plural sheets in the double-face printing operation, a part of the sentences corresponding to one paragraph is ended at an intermediate portion of a third page, and there is a blank on the remaining portion of the third page and a full blank on the next page (reverse face) and a next paragraph is started at a fifth page.
In summary, in the double-face printing operation of the conventional perfecting printer, a printing operation is promptly conducted on the reverse face of the sheet even if there is no print data therefor. Accordingly, a total printing period for printing only the print data for the obverse face on the sheet would be the same as that required for printing both print data for the obverse and reverse face on the sheet. Accordingly, the perfecting printer needs a surplus and unnecessary time to conducting the double-face printing operation on the sheets, at least one of which has a reverse face to be printed as a blank page.